In general, a welding target position, which is one of welding conditions, can be set based on, e.g., groove shape information. However, a shape of a weld bead does not always coincide with an estimated one, so that it is necessary to measure the welding target position from the shape of the weld bead before welding of a subsequent pass.
As a conventional technique, there is known a method that calculates a difference in cross section area of the weld bead between current and past shape data of the weld bead using a laser measurement system to control a welding speed of an automatic welding machine with a welding amount set as one of the welding conditions. Further, there is known another method that detects a changing point of a groove surface or the weld bead so as to control a target position. Further, as a method using an image, there is a method for dealing with a narrow groove, in which an end of the groove surface or weld bead bottom is detected through image processing to set the welding target position to a center of a groove width.
A technique described in Japanese Patent Application Laid-Open Publication No. 8-33979 (Patent Document 1), the entire content of which is incorporated herein by reference, uses an image of a groove shape obtained before or immediately after welding of a second or subsequent pass and an image of a groove shape obtained at the same position as that of a previous pass to perform image processing, calculates a contact point of a part departing from a part where the two images of groove shapes overlap each other, and determines the target position or the welding condition based on the calculated position. A technique described in Japanese Patent Application Laid-Open Publication No. 9-141439 (Patent Document 2), the entire content of which is incorporated herein by reference, is a welding method in which groove sensing is performed before a final pass operation of each layer.
However, as the welding target position, there can be considered not only a common V-shaped groove, but also an object to be welded having a wide groove width and a groove having a complicated shape changing in a three-dimensional manner. In such a case, a multiple pass per one layer welding method is adopted. In this method, the welding target position within the groove differs depending on a position of a welding pass, with the result that features on a weld bead thereof to be measured also differ.
In the above-mentioned welding techniques, it is difficult to set the welding target position of a subsequent pass due to application of the multiple pass per one layer welding method at a welding position or depending on a welding attitude at a welding position. The welding target position can be set based on groove shape information. However, as described above, a shape of the weld bead does not always coincide with an estimated bead shape as described above. This makes it difficult to measure the welding target position from the shape of the weld bead before welding of a subsequent pass and accurately select the welding target position.
Further, in the techniques described in Patent Documents 1 and 2, when the multiple pass per one layer welding is performed, a folding point is detected and the detected folding point is set as the welding target position, which poses a problem that it is not possible to obtain accurate welding target position information.